Check Completeness of a Binary Tree LeetCode solution in Java

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class Solution {
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  public boolean isCompleteTree(TreeNode root) {
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    boolean end = false;
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    Queue<TreeNode> queue = new LinkedList<>();
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    queue.offer(root);
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    while (!queue.isEmpty()) {
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      TreeNode currentNode = queue.poll();
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      if (currentNode == null) {
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        end = true;
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      } else {
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        if (end) {
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          return false;
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        }
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        queue.offer(currentNode.left);
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        queue.offer(currentNode.right);
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      }
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    }
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    return true;
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  }
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}